Bacterial cell-to-cell communication, termed quorum sensing (QS), leads to coordinated group behavior in a cell-density-dependent fashion and controls a variety of physiological processes including virulence gene expression. The repressor of the lsr operon, LsrR, is the only known regulator of LuxS/AI-2-mediated QS in Salmonella. Although lack of lsrR did not result in noticeable differences in Salmonella survival, the down-regulation of QS as a result of lsrR overexpression decreased Salmonella survival within macrophages. We found that impaired growth of Salmonella overexpressing lsrR within macrophages was due largely to its hypersensitivity to NADPH-dependent oxidative stress. This, in turn, was a result of decreased expression of genes involved in the oxidative stress response, such as sodA, sodCI, and sodCII, when lsrR was overexpressed. These results suggest that down-regulation of QS by excess LsrR can lower Salmonella virulence by hampering Salmonella evasion from oxidative killing within macrophages.